In some laser applications, such as optical communications, laser radar, and microwave generation by heterodyning of lasers, it is necessary or desirable to be able to simply detect or measure the difference between two laser frequencies by, for example, generating an analog signal that is a measure of the frequency difference. There are several methods of measuring the difference between two lasers; here is described a laser-difference-frequency discriminator that measures the difference between two laser frequencies and outputs a voltage at baseband that is related to the difference frequency. The discriminator should be capable of measuring changes in the difference frequency at rates approaching a megahertz, but the use of this discriminator is limited to difference frequencies that are within a photo detector's bandwidth. The concept for the difference-frequency-discriminator is related to delay-line frequency discriminators that are often used in the microwave region, but instead of bulking microwave delay lines, we use optics to give the laser-difference-frequency discriminator advantages in tunability, performance and size relative to a microwave delay-line discriminator. Laser-difference-frequency discriminators can be used, in conjunction with a feedback loop, to stabilize laser difference frequencies in such applications as laser radar and microwave generation.
The task of providing a laser-frequency discriminator system is alleviated somewhat by the systems disclosed in the following U.S. patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 4,982,082 issued to Ottusch, entitled “Frequency Detector for discriminating multi-longitudinal mode laser operation”.
U.S. Pat. No. 4,701,924 issued to Thomas, entitled “Frequency Discriminating Laser”.